Microsoft has released their adCenter Labs Beta with a few interesting tools. One of the interesting tools is their Search Funnel program that will allow you to see what keywords people searched prior to or after any search term you are interested. Here are some examples using the term "ipod nano":

I played around with the tool and it appears only go grab the most popular terms with significant traffic, so it won't be much use for true "tail" SEMs, but it can still come up with some interesting ideas and keyword nuggets you may not have thought of before.